In electric motors which have no coils or induction bands, successfully starting the motor under heavy loads can present some unique challenges. In the particular class of electric motors discussed herein, referred to herein as salient-poled-rotor electric motors, rotor torque is created by alternately creating a magnetic pole force in every other pole of the stator. For any three poles of the stator, the stator poles will alternate between a first state, wherein a pole force is exerted upon the rotor by the inner stator pole, but not by the outer two stator poles, and a second state, wherein a pole force is exerted upon the rotor by the outer two stator poles, but not by the inner stator pole. In such motors, the available torque between the rotor and the stator fluctuates between a minimum torque position, which nearly cancels all start torque, and a maximum torque position, which provides the greatest start torque, as each rotor pole moves in and out of register with respective stator poles. Thus, in developing starting torque, the position of the salient poles of the rotor in relation to the stator poles is the critical factor. The maximum torque position for the motor is to have the salient pole faces of the rotor positioned center of the stator pole slots, which places the face of each rotor pole halfway between two stator poles. In this position, each rotor pole face is half in register with one stator pole face and half in register with an adjacent stator pole face. The minimum torque position occurs where the rotor pole faces are all in register with a respective stator pole.
The cancellation of start torque in the minimum torque position is created by magnetic flux from the stator poles from which the rotor poles are not in register. Thus, the stator poles to the right and to the left of any rotor pole both exert an attractive force on the rotor pole which is in register with the stator pole between them. The result is that the attractive forces that would otherwise induce the rotor to move are canceled by one another. In this balanced condition, there is near equal force to induce clockwise rotation as there is force to induce counter-clockwise rotation. The result is often only a vibrating rotor unable to achieve enough torque to move against the load.
One known salient-poled-rotor electric motor is described in U.S. Pat. No. 6,787,958 to Walter, which is referred to herein as the Walter Motor. The Walter Motor attempts to provide adequate starting torque by using shaped rotor poles. In particular, one side of each rotor pole is slanted outward from the pole face to the pole root, thereby creating an asymmetrical registration between the rotor pole and the corresponding stator pole. Although this creates a slight unbalancing of the attractive magnetic forces when the rotor poles are fully registered with the stator poles, the neutral position has only been moved, not eliminated. Thus, the start torque minimum position appears when the rotor poles are slightly out of register with one set of the stator poles. Accordingly, this type of shaped pole motor is essentially mono-directional, because it creates poor starting torque and poor running characteristics in the direction of rotation opposite the direction in which the poles are slanted. Even at its best, the pole shaping method used by Walters provides poor starting torque characteristics in applications where the motor is started under load.
In salient poled rotor electric motors that utilize a control circuit having feedback sensors, the control circuit is sometimes able to start the motor on its own. This is particularly true where there is little or no starting load, as in fan applications. However, if the motor must be started under heavy load, previously known control circuits will not be able to start the motor if the rotor has stopped in the minimum torque position. Thus, if the motor is to be started under load, a positioning system must be provided for moving the rotor to the maximum torque position against the load before attempting to start the motor.
It would be desirable to energize the coils of the stator of salient-poled rotor electric motors to create a very strong positioning torque on the rotor poles which tends to center the rotor poles in the maximum torque position and can be controlled in such a way that the amount of positioning torque can be great or small depending on the starting load. It would also be desirable to have a start positioning system for salient-poled rotor electric motors which use an H-bridge circuit, and for salient-poled rotor electric motors which use a bifilar circuit. It would further be desirable to have a start positioning system that can be used with salient-poled rotor electric motors which run synchronously on alternating current without a run-circuit, wherein the start positioning system serves as the starting circuit for the motor to accelerate the rotor from stasis to synchronous operation against a load.